May I see you náked on webcam?<br />
If they block you, then they are a woman. If they say yes, then they are a guy.<br />
Isn't it pretty obvious o.O?
sniff his crotch
Kick 'em in between the legs, with a moderate amount of force. If they weep openly, whilst crawling on the floor, then they're probably male, lol!
Maybe ask htem if they have an inney or an outey?
Say you want to see their DL picture.